Setting up carbs, & ABS on Tohatsu 4 stroke 25 H/P MFS25A

[Bob Le]

I have had some problems with this engine over the years - mainly carb's going out of tune.
I have been able to re-establish a reasonable state of tune after much fine adjustment of the
three throttle stop screws, & the three idle mixture screws.

This year I had a bad missfire which took some time to isolate to the bottom cylinder -
eventually it revealed it's self, & changing the spark plug
has cured that problem (nothing wrong to see on the old plug)

The caps covering the idle mixture screws were removed years ago,
& I have re-tuned several times - all approx 2 turns out.
Just wondered what the official way was to set them.

The ABS unit on my engine has a fine male thread in the upper (active) part which screws into the lower part which contains a spring. I don't believe the two were originaly screwed tightly together - rather that the top part was "unwound" approx 2¼ turns. This is why I think that this is an adjustment of how long the choke is active for - ie the further out the top part is then the longer it will take for the needle, & outer cylinder to drop down, & seal off the starting jet.
I have checked out the ABS off the engine using a 12V supply, from cold to hot the needle moves by 6mm. This took about 3 mins to happen
Is there a measurement of how far the needle should extend beyond the plastic base when cold? this would allow correct setting on the fine thread.

I am in the UK by the way.
Regards
Bob.

 

[pvanv]

Is there a symptom now? A good ear, precision shop tach, and manometer can help you set the carbs -- just like any multi-carb setup. The ABS we get in the US is not adjustable... that time and throw sounds correct. If you need less dwell to the starting enrichment, you could experiment with unthreading the ABS halves... but how do you lock that adjustment in place?

 

[Bob Le]

Deleted

 

[Bob Le]

Paul.
Having changed the bottom spark plug the main problem has gone, but I have found over the years that I have had this engine
that it does go off tune from time to time.
This is noticable when manouvering into locks etc where to have an engine stall on you is not recommended !
I have always been able to re-tune - as you say just by listening to the engine note, & smoothness,
I have been doing this for decades on car engines.
I have used a manometer, but find that a short piece of hose, and listening to the "suck" volume on each carb
usually works well enough, and is much quicker.
I have a tacho on the boat which, while not precision, shows variations of speed well enough.
I find 1000 RPM to be a good idle speed.
I just wondered how the factory would have set the idle mixture.
I have the workshop manual, but of course it does not cover the carbs, as they are meant to be locked.

Re the ABS - the screw thread is plastic to plastic, and is quite tight - it would not shift once set.
Would it be possible to measure the needle protubance on a USA one when cold ? I could then set mine to be the same.

Thanks for you patience with this topic.

Best Regards
Bob.